This invention relates to a catalyst comprising carbon supports supported with carburized platinum and other metals, especially a platinum catalyst employed for the electrode catalyst of a fuel cell and a process for preparing the same.
Catalysts comprising carbon supports and various catalyst metals, mainly a platinum metal supported thereon have heretofore been employed as those for various chemical reactions and for the electrodes of fuel cells. A number of other catalysts which contain the other metals such as nickel and chromium in addition to the platinum have been known for promoting the catalyst performance.
The catalyst performance of these catalysts depends on the degree of dispersion these metals and the performance is promoted with the increase of the surface area of the catalyst. If the same amount of the catalyst is supported thereon. Temperatures at hitch the above catalysts are employed in the fuel cells and in the various chemical reactions are between 80.degree. and 210.degree. C. for the former and they may often exceed 210.degree. C. for the latter depending on the reaction. In such high temperatures the absorbing force between the support and the catalyst particles supported thereon is weakened so that the catalyst particles may move on the surface of the supports to agglomerate with each other. The agglomeration results in the decrease of the surface area, which brings about the decrease of the catalyst performance, to produce the drawback of the lowering of the cell voltage at a certain current density or the shortening of the catalyst life.
Various kinds of catalysts have been proposed as improvements of the above conventional catalysts. However, most of these catalysts attach importance to the catalytic activity itself and pay little attention to the extension of the catalyst life. In the U.S. Pat. No. 4,202,934 is described a catalyst which has been made by alloying platinum with vanadium for promoting the catalytic performance, which is reported to lose 67.5 weight % of the vanadium in48 hours from the beginning when employed as the cathode of a hot phosphoric acid type fuel cell (U.S. Pat. No. 4,316,944). As the improvement of the above catalysts use of chromium has been proposed as an alloy component. Also in this case. 37.5 weight % of the chromium is reported to dissolve out in 48 hours from the beginning when employed as the cathode of a fuel cell.
Further the U.S. Pat. No. 4,447,506 is described the addition of cobalt to a platinum-chromium alloy for improving the activity for reduction of oxygen in a hot phosphoric acid type fuel cell. However, no description about the life of the electrode catalyst under the operating conditions is included therein.
As to the problem of the surface area decrease of the platinum catalysts a method for depressing the surface area decrease by attaching carbon around the platinum by means of heat treatment of the catalyst in carbon monoxide has been proposed (U.S. Pat. No. 4,137,372). Also by this treatment, the surface area of the catalyst is shown to decrease to about half of the original value 100 hours from the beginning when employed as the cathode of a fuel cell.